This invention is an improvement of a recently introduced and unique in-die strand-quenching process disclosed in U.S. Pat. No. 3,981,959 issued Sept. 21, 1976, to the assignee of the present application, namely, Leesona Corporation. In such patent, a system which permits drastically increased pelletizing operation efficiencies by means of introduction of a liquid coolant under pressure into direct contact with the outer surface of a polymeric or other thermoplastic material passing through it, is disclosed. As the coolant enters the die orifice, a portion thereof is immediately vaporized to form at least a solidified outer wall of material surrounding an inner and still somewhat liquid material core. The wall of the orifice is simultaneously lubricated by non-vaporized coolant to facilitate its passage through the die. Such system, also contemplates particularly when utilizing polymeric materials, although not necessarily so, the subsequent cutting of the resultant strands into discrete material lengths or pellets by knives mounted for rotation in respect to the exit face of the die.
In addition to thermoplastic polymeric material such as polyethylene, polyproplylene, polyvinylchloride, etc., the process has applicability to other thermoplastic materials such as glass and various metals.
The above described system generally utilizes a porous member through which the liquid coolant is forced, such member being disposed in an intermediate zone of the extrusion orifice through which the material passes. Because of the high coolant efficiencies involved, portions of such member are necessarily cooler than the portions of the die forming the upstream die orifice. On some occasions the relative lower temperature of the porous member can be transmitted by conduction upstream to the oncoming material to an extent to permit a presolidification thereof sooner than desired. Additionally, upon start-up it takes some time for the various temperature gradients between various components making up the die to thermally equalize to a steady state operation. Fairly immediate achievement of such steady state operation is desirable so as to reduce throughput of unacceptable product upon start-up operations. It is accordingly an object of the present invention to overcome such potential operational aspects of the system in an inexpensive and straightforward manner.
A further object of the present invention is the provision of a method for insulating from conductive heat transfer, the initial and intermediate extrusion zones of the aforementioned system so that steady state operational temperatures can be rapidly achieved and sustained during start-up.
These and other objects of the present invention are accomplished by the provision of a novel extrusion method comprising the introduction of an insulative insert between the initial and intermediate extrusion zones of an extrusion system wherein heated thermoplastic material is forced into the initial zone of an extrusion die having at least one extrusion orifice, forcing said material through said orifice and outward of said die in rod or stand-like form while forming an at least solidified outer wall structure surrounding a more fluid inner core of said material at a higher temperature. In such procedure, a liquid coolant under pressure is forced into the intermediate zone of said extrusion so as to contact the outer surface of said material passing therethrough. The coolant is at least partially vaporized simultaneously to quickly form said solidified outer wall structure and to provide a lubricating film between the inner wall of said orifice material to facilitate passage of said material therethrough.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.